drm/amdgpu: add check for atombios GPU virtualization table

This table is found on boards that support SR-IOV.  This will
be used to determine if the board supports SR-IOV and allow
the driver to take specific action in certain cases.

Reviewed-by: Monk Liu <monk.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>